This is a common problem on anything that is aging and uses gas struts... as you already correctly said the lower temperatures mean the gas inside has lees push on the piston and hence a heavy roof/boot lid etc will tend to drop
there are a few tricks people use - Matt and a couple of others use a piece of 15mm copper pipe that can be put between wide lower part of the strut and the mount point on the body - hopefully they can put up a pic
Some folks use bits of timber to keep the roof held up - even in summer for the really bad ones!
I use an old broom handle on my tailgate
The only way to sort it out properly is to buy new struts
